1. Field of the Invention
The present invention relates to a ball screw mechanism for a type of electric power steering apparatus which assists the movement of a rack shaft by an electric motor, more particularly relates to a ball screw mechanism used for transmitting a driving force of the electric motor to the rack shaft as a steering shaft in the electric power steering apparatus.
2. Description of Related Art
Conventionally, in a so-called rack-assist-type electric power steering apparatus, the movement of the rack shaft in the axial direction is assisted by a driving force of an electric motor. In this type of electric power steering apparatus, a ball screw mechanism is constructed by forming a ball screw portion on the rack shaft and fitting a ball nut on an outside of the ball screw portion, so that the torque generated by the electric motor is transmitted to the rack shaft through the ball screw mechanism.
A screw groove is formed on the inner circumference of the ball nut, and a groove path formed by a combination of the screw groove and the ball screw portion of the rack shaft is filled with a plurality of balls. In such a construction, smooth rotation of the ball nut is ensured, and the rack shaft is moved in the axial direction as the balls move in the groove path with the rotation of the ball nut. Accordingly, since the ball nut can be rotated by the electric motor by attaching a large gear such as a bevel gear to the outer circumference of the ball nut and engaging a small gear attached to the motor shaft of the electric motor with the large gear, it is possible to assist steering by moving the rack shaft in the axial direction with the rotation of the ball nut.
Moreover, in the ball nut, in order to circulate the filled balls in a portion called a ball screw track within the groove path, a circulator is attached to the outside of the ball nut. The circulator is a tube-like member for returning the balls discharged outside the ball nut from one end portion of the ball screw track to the other end portion of the same inside the ball nut by transferring the balls via the outside of the ball nut. In other words, the groove path between two ball circulating holes for connecting both end portions of the circulator to the groove portion inside the ball nut becomes the ball screw track.
Note that, as a steering apparatus of a type different from the above-mentioned rack-assist-type electric power steering apparatus, for example, Japanese Patent Application Laid-Open No. 2002-87288 discloses an apparatus using a ball screw mechanism for transmitting a driving force between a steering wheel shaft to which a steering wheel is attached and a cross shaft connected to a steering mechanism. In this conventional steering apparatus, the groove path except the ball screw track in which the balls are actually circulated of the ball nut of the ball screw mechanism incorporated in the apparatus is not polished, and thereby preventing an outflow of balls from the ball screw track.
By the way, as described above, the ball screw mechanism needs to circulate the balls within the range of the ball screw track by the circulator. Therefore, when assembling the ball screw mechanism, there is the problem that the balls sometimes flow to the groove path outside the range of the ball screw track. Since the balls flowed to the groove path outside the ball screw track may interfere with the rotation of the ball nut, the flowed-out balls need to be discharged from the ball nut.
As a structure for discharging the balls flowed to the groove path outside the ball screw track from the ball screw mechanism, conventionally, in general, the ball nut is formed with a so-called open structure in its end portion. However, when the open structure is adopted, it is necessary to increase the outside diameter of the ball nut to ensure rigidity thereof, and consequently the whole ball screw mechanism becomes larger and there is the problem that the increase of the size of the ball screw mechanism does not meet the current situation where a reduction in the size of the apparatus is required. Moreover, in the ball screw mechanism incorporated in the electric power steering apparatus, since the ball nut is rotated at high speed by driving the electric motor, when the outside diameter of the ball nut is increased, the moment of inertia is considerably increased. Thus, there is a problem of deterioration of steering feeling in an electric power steering apparatus incorporating such a ball screw mechanism.
Further, when the open structure is not adopted for the ball nut, the balls flowed to the groove path outside the ball screw track will stay in the groove path outside the ball screw track. In this case, in some rotation state of the ball nut, there is a fear that the balls flowed to the groove path outside the ball screw track may push an end portion of the circulator and interfere with smooth circulation of the balls.